Simulated moving bed chromatography (SMB) utilizes a number of interconnecting separation beds containing solid phase chromatography substrates. Inlet ports for feedstock and desorbent and outlet ports for raffinate and extract are placed at specific points in the series of separation beds, and a series of valves and tubing and/or channels connects flow to provide a continuous loop. Liquid flow is controlled by two or more pumps connected to the inlet and/or outlet streams. At defined intervals, the positions of the inlet and outlet ports are switched in the same direction as the flow, simulating a countercurrent movement of the solid phase beds relative to the fluid streams. Feedstock introduced into the first bed begins to separate into components contained therein as flow ensues, with less retained species migrating in the direction of fluid flow and being collected at the raffinate port. The more retained species remain preferentially associated with the solid phase and are collected at the extract port. By regulating the switch times and flow rates of feedstock, desorbent, raffinate, and extract, a standing wave pattern is established, allowing for continuous flow of purified products from the system.
The principle of continuous countercurrent chromatography relies on the phenomenon of preferential retention on an immobilized sorbent substrate of one or more substances in a feedstock mixture, separation of less retained substances, and subsequent recovery of the separated substances. In standard SMB, this process is repeated in a succession of columns by switching zones of separation, enrichment, and regeneration in stepwise sequence using a valve system.
For large scale industrial systems, the bed volume is so great compared to void volumes of liquid between beds that even elaborate valve systems involving extensive conduits do not interfere with the process. There has been a recent trend, however, in scaling SMB down to pilot and sub-pilot volumes, as the need for more sophisticated applications has arisen in the fine chemicals and pharmaceutical fields requiring milligram-to-gram level quantities of product. For example, the Protein Structure Initiative is a national effort to determine the three-dimensional structure of a wide variety of proteins. This initiative will accelerate the discovery of protein function and enable faster development of new therapies for treating genetic and infectious diseases. One of the significant challenges is to develop methods of purifying target proteins from complex cell extracts in small (10-100 milligram) quantities, in high purity (greater than 90%). SMB scaled down in size promises to provide a mechanism for overcoming these challenges.